In the soluble coffee art there has been a great deal of emphasis upon maximizing the soluble solids yield from roasted and ground coffee, most notably by varying percolation conditions. Early on in the development of instant coffee, in the pre-World War II period, solubles were leached out of roasted and ground coffee with boiling water to yield less than a 25% soluble solids yield. Morganthaler in U.S. Pat. No. 2,324,526 utilized temperatures of from 320.degree. to 347.degree. F. to achieve 27 percent solubles. Sivetz and Desrosier in "Coffee Technology," Avi Publishing Company, Inc., 1979, set forth the state of the art for solubles yield at page 366. Therein, it is stated that overall soluble coffee yields from roasted and ground coffee are about 40 to 50 percent and that higher yields "are not feasible in the percolator without causing compression of grounds, reducing flow rates...and otherwise jeopardizing productivity and quality," the quality of the soluble coffee already being jeopardized at the upper end of this yield range by the generation of tars and other objectionable flavors. In view of the recognized limitations on percolation, attention has been focused on other methods for increasing soluble coffee yields.
Acid-catalyzed hydrolysis of partially extracted coffee grounds to obtain an increased solids yield has been examined as one means for increasing soluble solids yield. For example, U.S. Pat. No. 2,573,406 to Clough et al. discloses a process for producing a soluble coffee which involves atmospherically extracting about 20% of the weight of the coffee, hydrolyzing a portion of the grounds in a suspension of about 1% sulphuric acid at 100.degree. C. for about one hour, adjusting the pH of the hydrolysate, filtering the hydrolysate, combining the same with the atmospheric extract and drying the combined extract. In another, similar process described in U.S. Pat. No. 2,687,355 to Benner et al., phosphoric acid is used in place of sulphuric acid. In still another process, disclosed in U.S. Pat. No. 3,224,879 to DiNardo et al., either alkaline or acid hydrolysis is carried out directly in the extraction train on coffee grounds that have been at least atmospherically extracted. Hydrolysis directly in the extraction train eliminates the separate hydrolysis step of the prior art processes and provides for adsorption of the alkaline or acid catalyst in the mass of spent coffee grounds.
More recently, Fulger et al. in U.S. Pat. No. 4,508,745 disclose a method for hydrolyzing a coffee extraction residue material to produce mannan oligomers from DP 1 to DP 10 by preparing a slurry of spent grounds at a concentration of 5% to 60% by weight, adjusting the pH to about 0.5 to 4.0, and reacting the slurry at a temperature of 160.degree. C. to 260.degree. C. for 6 seconds to 60 seconds. According to Fulger et al., the aforesaid method can achieve a soluble yield increase on the order of 30% by weight from a coffee extraction residue material, said residue material having been partly extracted, as for example the spent grounds from a commercial percolation system that have been atmospherically extracted and partly thermally hydrolyzed.
However, in some instances it may be undesirable or impractical to employ an acid-catalyzed reaction to achieve an increased soluble yield. Thus, it is an object of the present invention to provide a method for treating roasted and ground coffee in the absence of an acid catalyst to achieve an increased soluble yield.
It is a further object of the invention that the method be efficient and that the soluble solids generated by the method be of an acceptable flavor quality.